MPI_Win_flush completes all outstanding RMA operations initiated
by the calling process to the target rank on the specified window. The operations
are completed both at the origin and at the target. MPI_Win_flush_all completes
all outstanding RMA operations to all targets.

Almost all MPI routines return an error value;
C routines as the value of the function and Fortran routines in the last
argument.

Before the error value is returned, the current MPI error handler
is called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler;
the predefined error handler MPI_ERRORS_RETURN may be used to cause error
values to be returned. Note that MPI does not guarantee that an MPI program
can continue past an error.